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1947-2014 (Archived)

The most amazing thing about memory is how precisely we forget. Our brain retains only what it predicts will be important in the future and forgets the rest. There is no point in remembering where you parked your car at Wal-Mart last February -- unless it was stolen. That would be unforgettable. Scientists have long known how the brain predicts which experiences to retain in long-term memory and which ones to let fade away. But now they have made a new discovery: why we often remember useless stuff.

The first rule of learning is repetition. Repeating something over and over, as you did to learn your multiplication tables, moves memory from temporary short-term storage into permanent long-term memory. This is because the brain views something that is encountered repeatedly as more likely to be important to the person (or animal) in the future.

The second way events get seared permanently into memory is if they are associated with extremely strong emotional reactions, as would happen if, upon emerging from Wal-Mart with your shopping goodies, you were to find your car gone. This is because, in evolutionary terms, an organism shouldn't risk repeating a stressful, potentially life-threatening experience to remember it.

In the last 15 years, neuroscientists have determined the cellular and molecular mechanisms for how these two kinds of experiences are moved from short-term memory into long-term memory. But memory researcher Richard Morris of the University of Edinburgh noticed something about memory that is not explained by these well-accepted rules and molecular mechanisms of memory. Our minds are filled with scraps of completely irrelevant information. This includes snippets of experiences that were neither repeated nor associated with a traumatic event. Indeed, they are useless and would be better forgotten, but they persist nevertheless. How these remnants of trivial memories are retained cannot be explained by the detailed molecular mechanisms that have been carefully worked out in studies of memory in laboratory animals.

The answer is found in another factor that helps the brain predict whether or not an experience should be saved in long-term memory: novelty. When our daily routine is suddenly disrupted by an experience that is truly novel, the mind "perks up." It makes good sense to activate the long-term memory mechanism in this case, because a new experience is likely to provide important new information that will be useful to an individual in the future, and so the experience should be added to the long-term memory store. In the brain, novelty is signaled by neurons that use the neurotransmitter dopamine. Dopamine circuits do not code sensory perceptions; instead they rev up the level of activity broadly across neural networks in the brain.

In their experiments the researchers found that if they used an exciting, novel experience to disrupt a rat's training to find a food reward, the rats remembered where the food was hidden, and this memory did not fade away as it did in other rats that had gone through the same rigorous training routine but without the novel interruption. The novel experience that the scientists used to interrupt the training session was simply treating the rat to an excursion to new cage. This, in contrast to the life of confinement in its home cage, was a thrilling expedition.

That novel experience had nothing to do with the skill the rat was learning in its training sessions, so why did the novel stimulus improve the rat's performance in the maze? Studying the molecular changes in neural circuits storing memories, the researchers discovered that the molecular machinery known to store long-term memories had been switched on by the novel experience. The long-term memory mechanisms were activated by the surge in dopamine activity coding the experience as novel. That rat will never forget its stimulating exploration of the new environment. But, these changes inside the neurons that started the molecular machinery working were not yet idled by the time the rat was subjected to the next round of training in the maze. Thus, along with all the novel and unforgettable sights and smells and experiences of the novel outing, where the food was hidden in the maze was also permanently embossed in its long-term memory. The researchers found that the long-term memory storage mechanisms simply take a few hours to cool down.

The scientists found that the novel experience increased the rat's memory of the maze even if it followed the training session, simply because the rat's short-term memories from the training session were still being held in the brain temporarily while the animal was in the novel environment, so they too got stored permanently with all the other short-term memories before they faded.

This could explain how "useless" scraps of information in your mind might have gotten stuck there. They could have been surrounded by some truly novel experience that had nothing at all to do with the memory. But this new finding can also be put to advantage. While the ancient methods of repetition and punishment to drum information into a schoolkid's mind can be effective, so too should breaking up the doldrums of a lesson with a fascinating new experience that is completely unrelated to the lesson. The student union might be as important to long-term learning as the campus library.

This unforgettable information is published in the Nov. 9 edition of the Proceedings of the National Academy of Science, in a paper by Wang, Redondo, and Morris.

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